1. Field of the Invention
This invention relates generally to hydrocarbon exploration and extraction and, more particularly, the invention relates to a method for removing or eliminating water and/or gas condensates blockage in well bores and at the face of hydraulic fractures by using the wetting and related interfacial properties of various oil forms in order to decrease the forces of capillary retention of water and/or gas condensates and thereby increase the efficiency of extraction of hydrocarbons from underground reservoirs.
2. Description of the Prior Art
Accumulation of water around a wellbore, commonly described as water block, is widely recognized as a serious cause of decrease in production of hydrocarbon fluids which include both oil and gas and also liquid hydrocarbon condensates. Water blocking can be defined as the consequence of higher water saturation around the near-well-bore formation compared to the connate water saturation associated with rock that is more distant from the wellbore.
Water blocking can arise from any one or a combination of the following conditions, drilling, completion, fracturing, work over, or casing leaks. Because of the higher water saturation in the rock around the wellbore, the productivity of gas or oil flow is reduced. The presence of high water saturation in the wellbore region greatly increases the transmissibility of water into the well and greatly reduces the transmissibility of oil and/or gas. The effect on transmissibility is expressed through increased relative permeability to water and reduced relative permeability to gas and/or oil. The local change in transmissivity results in drastic suppression of the ability of oil or gas to flow into the well from the surrounding formation.
Even under conditions of flow of both phases or flow of hydrocarbon gas and/or oil, high water saturation is maintained around the well bore because of capillary forces. The problem becomes increasingly serious with decrease in permeability of a formation because pore sizes are smaller and capillary action is stronger.
The problem of water blocking is further accentuated if the well bore region has been subject to clay particle invasion or clay swelling. Dispersion, migration, and plugging by fine particles during well drilling and completion operations results in smaller pore sizes around the well bore. The locally decreased pore sizes result in impaired permeability and exacerbate the problem of water block by capillary retention. Damaged zones are prevalent in the perforation regions where access to the formation through well casing has been established by means of explosive shaped charges which crush the rock. Crushing of the rock in the local region of the perforation results in reduced pore size with attendant reduced permeability and increased capillary retention of water.
The clean up or removal of water blocking is currently difficult, expensive, and time-consuming. One approach to mitigating this problem is to treat the well bore with chemicals that adsorb onto the rock surface to render it near neutral to oil wet. Capillary retention forces are reduced and permeability to hydrocarbon is increased. The chemicals proposed for wettability alteration are expensive. Furthermore they may only be effective for a limited time because they are generally held as a monolayer at the rock surface and the wettability alteration may not be stable over time at the prevailing flow conditions.
Some kind of chemicals can be used to enhance the cleanup of the water block. Alcohol can be used to mitigate water blocking of gas production. The effects of various liquids such as brine, alcohols, and toluene on the gas deliverability have been studied. From this, it has been concluded that the cleanup of water block near a well bore could be divided into two stages. The first stage was fluid displacement which bypassed water and left high-retained water saturation around the well bore. This stage lasted about two days. The second stage was reduction of water saturation by evaporation driven by gas flow (mass transfer of water into the gas phase as the gas expands during flow into the well bore). The second stage could last for several months. The addition of volatile solvent helped the remediation in the second stage and reduced the duration of the cleanup.
It has also been found that the addition of methanol could speed the cleanup of water blocking. Wettability change from water wet to oil wet through using 1% v/v solution of octyldecyltrichlorosilane (OTS) could also aid in the clean up of water blocking in limestone when methanol was used to displace the water block. They mentioned that surfactants which change wettability might be used to enhance clean up of water blocks in tight gas sands.
However, if the draw down pressure were much higher than the capillary pressure of the near-well bore formation, the water block would be removed. Under these conditions there was not much benefit to well treatment with alcohol or alcohol/surfactant to remove water block.
Others have concluded that change in wettability of the rock surface from water wet to oil wet would promote mitigation of water blocks because the capillary pressure and hence capillary retention of water was reduced. Non-emulsifiers (surfactants) dissolved in methanol were employed to induce wetting change from water-wet to non-wet (contact angle ˜90°).
Others have claimed that oil-based mud filtrate, which contained asphalt that was soluble in crude oil or aromatic solvent, significantly increased gas well deliverability and oil or condensate productivity compared to water-based fluids.